Fifty-six workers were exposed to radioactive wastes from a Japanese nuclear power plant in an accident that was kept secret for more than a month, officials acknowledged today.

The announcement by the Japan Atomic Power Co. was the first indication that a large number of persons may have been contaminated. An earlier report had indicated that at most 16 may have been affected.

The latest disclosure of mishaps at the Tsuruga nuclear power plant raised new uncertainties about nuclear power development in Japan, where construction of new plants has been stalled, in part by the widely publicized incident in 1979 at Three Mile Island in Pennsylvania.

Officials described the extent of contamination of the workers as "negligible" but the nature of the accident and the lengthy secrecy surrounding it transformed it into a major affair in nuclear-conscious Japan.

Officials at the Ministry of International Trade and Industry, which is in charge of the investigation, said they could not yet gauge the seriousness of the accident because they encountered new disclosures from the plant's management almost every day.

The accident occurred on March 8 when a large amount of waste water overflowed a filter tank at the plant, which is located on the Japan Sea west of Tokyo. At first, it was reported that the leakage was caused by a structural defect. Then it was learned that a valve had been left open permitting more than 40 tons of waste water to flow out.

The value was open for about three hours, officials acknowledged. In an attempt to stop the flow out of the tank, workers at the plant and from contracting companies tried to scoop some of it up in polyester buckets, according to the officials.

The accident was known to investigators this week only after significant amounts of radioactivity were discovered in samples of mud taken from a sewage outlet, which carries water into nearby Urazoko Bay. The water near the plant are a rich fishing ground.

It was the second time this year that the Tsuruga plant has been found to have concealed accidents that resulted in radioactive leakage. In January, there were two incidents of leakage caused by cracks in a water heater.

Officials moved quickly to investigate the latest incident, although it is not likely to change the government's position, which is one of encouraging nuclear power development in order to lessen this country's dependence on oil.

Business groups in Japan are strongly in favor of building more power plants. At present, 21 nuclear power reactors are in operation and an additional 14 reactors are planned by the end of 1985.